Imagine the Universe News - 26 June 2003

Rosetta Stone Decodes Gamma-Ray Burst Mystery

A computer animation of a gamma-ray burst/supernovae viewed from a distance. (3.1 MB QuickTime movie)(Description)

(Credit: NASA/Skyworks Digital)

Scientists have pieced together the key elements of a
gamma-ray burst, from star death to dramatic black hole
birth, thanks to a "Rosetta stone" found on March 29, 2003.

This telling March 29 burst in the constellation Leo, one of
the brightest and closest on record, reveals for the first
time that a gamma-ray burst and a supernova -- the two most
energetic explosions known in the Universe -- occur
simultaneously, a quick and powerful one-two punch.

The results appear in the June 19, 2003 issue of Nature. The burst
was detected by NASA's High-Energy Transient Explorer (HETE)
and observed in detail with the European Southern
Observatory's Very Large Telescope (VLT) at the Paranal
Observatory in Chile.

"We've been waiting for this one for a long, long time," said
Dr. Jens Hjorth, University of Copenhagen, lead author on one
of three Nature letters. "The March 29 burst contains all the
missing information. It was created through the core collapse
of a massive star."

The team said that the "Rosetta stone" burst also provides a
lower limit on how energetic gamma-ray bursts truly are and
rules out most theories concerning the origin of "long
bursts," which astronomers classify as those that last
longer than two seconds.

A supernova is associated with the death of a star about
eight times as massive as the Sun or more. Its core
implodes, forming either a neutron star or (if there is
enough mass) a black hole. The star's surface layers blast
outward, forming the colorful patterns typical of supernova
remnants. Scientists have suspected gamma-ray bursts and
supernovae were related, but they have had little
observational evidence, until March 29.

"The March 29 burst changes everything," said co-author Dr.
Stan Woosley, University of California, Santa Cruz. "With
this missing link established, we know for certain that at
least some gamma-ray bursts are produced when black holes, or
perhaps very unusual neutron stars, are born inside massive
stars. We can apply this knowledge to other burst
observations."

GRB 030329, named after its detection date, occurred
relatively close, approximately 2 billion light years away
(at redshift 0.1685). The burst lasted over 30 seconds.
GRB 030329 is
among the 0.2% brightest bursts ever recorded. Its afterglow
lingered for weeks in lower-energy X-ray and visible light.

With the VLT, Hjorth and his colleagues uncovered evidence in
the afterglow of a massive, rapidly expanding supernova
shell, called a hypernova, at the same position and created
at the same time as the afterglow. The following scenario
emerged:

A bluish Wolf-Rayet star -- containing about 10 solar masses
worth of helium, oxygen and heavier elements -- rapidly
depleted its fuel, triggering the Type Ic supernova / gamma-
ray burst event. The core collapsed, without the star's
outer part knowing. A black hole formed inside surrounded by
a disk of accreting matter, and, within a few seconds,
launched a jet of matter away from the black hole that
ultimately made the gamma-ray burst.

The jet passed through the outer shell of the star and, in
conjunction with vigorous winds of newly forged radioactive
nickel-56 blowing off the disk inside, shattered the star.
Meanwhile, collisions among pieces of the jet moving at
different velocities, all very close to light speed, created
the gamma-ray burst. This "collapsar" model, introduced by
Woosley in 1993, best explains the observation of GRB 030329,
as opposed to the "supernova" and "merging neutron star"
models.

This computer simulation shows the distribution of particles moving near light speed in the jet as it breaks out of the star. (1 MB QuickTime movie)(Description)

(Credit: Weiqun Zhang and Stan Woosley)

"This does not mean that the gamma-ray burst mystery is
solved," Woosley said. "We are confident that long bursts
involve a core collapse, probably creating a black hole. We
have convinced most skeptics. We cannot reach any conclusion
yet, however, on what causes short gamma-ray bursts."

Short bursts might be caused by neutron star mergers. A
NASA-led international satellite named Swift, to be launched
in January 2004, will "swiftly" locate gamma-ray bursts and
may capture short-burst afterglows, which have yet to be
detected.

The VLT is the world's most advanced optical telescope,
comprising four 8.2-meter reflecting Unit Telescopes and, in
the future, four moving 1.8-meter Auxiliary Telescopes for
interferometry. HETE was built by MIT as a mission of
opportunity under the NASA Explorer Program, with
collaboration among U.S. universities, Los Alamos National
Laboratory, and scientists and organizations in Brazil,
France, India, Italy and Japan.